The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention may find further application in quality control inspections, spectroscopy, and the like.
Conventionally, magnetic resonance systems generate a strong, temporally constant main magnetic field, commonly denoted B0, in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object. Nuclear spins of the object then possess a macroscopic magnetic moment vector preferentially aligned with the direction of the main magnetic field. In a superconducting annular magnet, the B0 magnetic field is generated along the longitudinal axis of the cylindrical bore, which is typically assigned to be the z-axis. In an open system, the B0 magnetic field is typically oriented vertically between a pair of pole pieces, which is again assigned to be the z-axis.
To generate a magnetic resonance signal, the polarized spin system is excited at resonance by applying a radio frequency (RF) magnetic field B1, with a vector component perpendicular to that of the B0 field. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis, the precessing magnetic moment generates a magnetic resonance signal at the Lamor frequency which is received by the same or another radio frequency coil in a reception mode.
Traditionally, RF receiver coils have been utilized with magnetic resonance imaging and spectroscopy equipment in either quadrature mode or phased array mode. Quadrature coils typically include at least two coils or coil arrays which view the same region of interest, but are sensitive to signals 90xc2x0 out of phase, such as a vertical field and a horizontal field. Analogously , birdcage coils, which are circularly polarized, have taps for two 90xc2x0 out of phase output signals. Typically, the 90xc2x0 offset signals from the two coils or coil arrays are connected to an analog phase shifting circuit which causes both signals to have the same phase. Phase shifting and summing the signals typically provides a signal to noise improvement of about the square root of 2. Quadrature mode is preferable where a limited number of channels exists and speed of reconstruction is important.
Alternately, the receiver coils may be operated in a phased array mode in which the 90xc2x0 offset signals are each forwarded individually to separate receivers. Operation in phased array mode is preferable where improved image quality is important, such as in transverse or coronal scans. Prior art coils either make the quadrature combination on the coil in quadrature mode or output multiple signals to multiple receivers in phased array mode without the ability to switch from one mode to the other.
The present invention contemplates a new and improved radio frequency receiver assembly which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a magnetic resonance apparatus includes a main magnet which generates a main magnetic field through an examination region. A radio frequency (RF) transmitter coil positioned about the examination region excites magnetic resonance dipoles therein. An RF transmitter drives the RF transmitter coil. A multi-mode RF receiver coil assembly receives magnetic resonance signals from the resonating dipoles and at least two receivers receive and demodulate output signals from the receiver coil assembly. The receiver coil assembly includes at least one first RF coil which is sensitive to a magnetic field along a first axis. The receiver coil assembly further includes at least one second RF coil which is sensitive to magnetic fields along a second axis which is orthogonal to the first axis. A signal combining circuit which is operatively connected to the first and second RF coils has a quadrature combining mode in which it quadrature combines signals received by the first and second RF coils and a phased array mode in which it passes signals received by the first and second RF coils to corresponding receivers without combining the signals. A switch assembly is connected to the signal combining circuit. The switch assembly switches the combining circuit between the quadrature combining mode and the phased array mode.
A multi-mode magnetic resonance method includes generating a main magnetic field through an examination region and transmitting RF signals into the examination region to induce magnetic resonance in nuclei. The induced magnetic resonance signals are received using a first RF coil and a second RF coil. The received magnetic resonance signals are phased shifted. One of a quadrature combination mode and a phased array mode is selected. In the quadrature combination mode, the phased shifted received magnetic resonance signals are combined, while in the phased array mode, the received magnetic resonance signals are passed uncombined. The received magnetic resonance signals are demodulated and reconstructed into an image representation.
In accordance with another aspect of the present invention, a multi-mode RF assembly for use in a magnetic resonance apparatus includes a first RF coil assembly comprising at least one RF coil which is sensitive to a magnetic field along a first axis to generate a first resonance signal. A second RF coil assembly comprising at least one RF coil is sensitive to a magnetic field along a second axis which is orthogonal to the first axis to generate a second resonance signal 90xc2x0 out of phase from the first resonance signal. A phase shift circuit shifts a relative phase of the first and second resonance signals by 90xc2x0. A signal combining circuit combines the phase shifted first and second resonance signals. A switch assembly switches between outputting a combined signal and the first and second resonance signals.
In accordance with another aspect of the present invention, a method of quadrature operation in a magnetic resonance apparatus includes generating a temporally constant magnetic field through an examination region and transmitting RF signals into the examination region to induce magnetic resonance in nuclei. Induced magnetic resonance signals are detected in quadrature using a quadrature coil assembly. The detected quadrature signals are phase-shifted by 90xc2x0 and combined into a quadrature signal and an anti-quadrature signal using a quadrature adder. The quadrature and anti-quadrature signals are transferred to a pair of receivers and reconstructed into an image representation.
One advantage of the present invention is that it provides switching between quadrature combination mode and phased array mode depending on the type of examination.
Another advantage of the present invention is that it uses the anti-quadrature signal from a quadrature combiner to improve image quality.
Another advantage of the present invention resides in use of quadrature mode for applications which require faster reconstruction speed.
Yet another advantage of the present invention resides in use of phased array mode for applications which require better image quality.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.